Everyone needs to stop using the weather as a scapegoat for disappointing economic data, says one economist.

The Institute for Supply Management said Monday its broad manufacturing index tumbled to 51.3, well below economists’ expectations and down from 56.5 a month earlier. The drop marked only the fourth time in the past 20 years that the index dropped by more than five points, according to Michael Feroli, chief U.S. economist at J.P. Morgan Chase.

Unusually bad weather reduced orders, or so the survey’s respondents said. The ISM noted that “a number of comments from the panel cite adverse weather conditions as a factor negatively impacting their businesses in January, while others reflect optimism and increasing volumes in the early stages of 2014.”

Mr. Feroli is skeptical of that excuse. His statistical analysts shows weather accounted for 0.1 point of the 5.2-point decline last month. In other words, “essentially none,” he says.

“We think there are some more plausible stories that can explain part of the weakness,” he wrote in a note to clients on Monday. “First, the ISM had generally looked too strong relative to some other indicators. Second, inventories had been running hot so some pullback in manufacturing makes sense.”

The inventory index dropped three points to 44.0, which he pointed to as a reason backing up his thesis.

“Even with these explanations, though, today’s ISM is a disappointment,” he says.

Thus, the sour market reaction. The Dow Jones Industrial Average recently plumbed new lows, down 250 points, or 1.6%, to 15449. The S&P 500 sank 1.8% to 1750 and the Nasdaq Composite fell 2.4% to 4007.